The black cab used by John Worboys. Photograph: Metropolitan Police/PA

One of Britain's most prolific sexual predators was allowed to remain free to drug, rape and assault more than 100 women over six years after police repeatedly failed to respond to the complaints of his victims.

John Worboys, 51, a licensed London cab driver, was told yesterday he faces a "very substantial" jail sentence after a jury convicted him of 19 charges of drugging and sexually assaulting 12 of the women, in one case raping his victim.

Police were last night bracing themselves for more women to come forward. They have received 85 complaints so far and believe that over his 13-year career as a London taxi driver he could have drugged and attacked more than 100 female passengers.

The watershed case has exposed serious failings in the way police treat allegations of rape and sexual assault and comes despite years of high-profile policies and promises to improve rape conviction rates that stick at less than 6%.

Campaigners said the details of how police failed to apprehend Worboys for six years, despite receiving numerous complaints from women, exposed the fact that frontline officers remained sexist, dismissive of allegations of sexual assaults and ultimately guilty of "sabotaging" rape inquiries.

One victim, who was drugged and assaulted by the black-cab driver when she was a 19-year-old student, told the Guardian: "This case raises serious issues that must be addressed. There needs to be scrutiny of the police and how they dealt with these matters."

The Independent Police Complaints Commission has begun an inquiry into why the Metropolitan police arrested Worboys and released him without charge in July 2007. He went on to attack another 29 women before he was finally arrested and charged in February last year.

Scotland Yard announced that after reviewing the case there will be a fundamental change in the way rape and serious sexual assaults are investigated. In future the investigation of all rapes will come under the control of a centralised unit and no longer be run by local borough commanders.

"It's clear more should have been done and he should have been caught earlier," a senior source said. "This man had a fairly unique method of operation, and it should have been picked up much quicker."

Worboys, a former porn actor and stripper from east London, hid behind the facade of a respectable taxi driver to pick up professional women from the centre of London and Bournemouth in the early hours of the morning, targeting victims who looked like they had been drinking.

He offered them a glass of champagne in his cab, claiming he had won money on the lottery or in a casino. The drinks were spiked with a powerful prescription drug and an over-the-counter-medicine, a cocktail which left the women so incapacitated he was able to rape and sexually assault them. When they woke the following day many of the victims could not remember what had happened.

Before Worboys was finally arrested last year, 14 women had made complaints to police over several years that they had been attacked or had an unsettling experience in a black cab. But officers failed to see a pattern. The complaints began in 2002 and by 2006 Worboys was using an identical method on all his victims.

Their accounts to police were sometimes hazy - details which experienced detectives say should have raised immediate alarm bells about date rape drugs, but the women's allegations were either marked as no crime, a crime-related incident or not properly investigated.

After Worboys was finally arrested and charged another 71 women came forward to say they had also been attacked by him. One victim, Carrie Symonds, waived her right to anonymity and said Worboys was "obviously a very sad, deluded person". She added: "I hate that it's being played down that he's just a weirdo. He's more than a weirdo, he's very, very, very dangerous."

Ruth Hall, from Women Against Rape, said: "Until people are held to account and sacked for not doing their job properly cases like this will continue to happen. The sexism and hostility to women who suffer rape and sexual assaults runs so deep officers will continue to sabotage rape cases, because this is what they are doing."

The failings come nine years after the Met created specialist teams - known as sapphire units - to investigate rape allegations. Deborah Glass, of the IPCC, said there were "very specific" concerns about the investigation by officers in Plumstead, south-east London, into the cabby's arrest in 2007. She said: "It is clearly concerning that, despite the Metropolitan police arresting Worboys in the summer of 2007, he went on to attack more women."

Last year John Yates, assistant commissioner of the Met and the Association of Chief Police Officers lead on rape and sexual assault, admitted that police officers were contributing to the "appalling" conviction rate in rape cases.

Yates said detectives did not apply the same professionalism to rape as they did to other crimes and blamed police for too often greeting complainants with scepticism and inertia. He said that officers "must absolutely accept the victim's version of events unless there are very substantial reasons to do otherwise".